Here's a short video of Eubranchus, the nudibranch featured in the Seaweed Microcosm post, below. Note the tiny swimming crustacean that puts in a brief appearance, about 9 seconds in from the start.
For more on British species of nudibranch, visit http://www.seaslug.org.uk/nudibranchs/
where you can read all about them and access pictures of all the British species
For pictures of their exotic, gaudy tropical cousins, see